1. Field of the Invention
The present invention relates to a toner used for developing an electrostatic latent image formed by an electrophotographic method, an electrostatic recording method or the like, and more particularly concerns a color toner that is suitable for a non-contact heat fixing system.
2. Description of the Related Art
With respect to the method for heat-fixing a toner image on copying paper, there are basically two methods, that is, a contact heat fixing system and a non-contact heat fixing system. The non-contact heat fixing system is a fixing system in which upon fixing, no members contact an image made from toner powder, and mainly classified into a flash fixing system and an oven (atmosphere) fixing system.
In the flash fixing system, a powder toner image, transferred onto copying paper from a photosensitive member or an intermediate transferring member, is irradiated with flash light from, for example, a xenon lamp, a halogen flash lamp or the like so that the toner image is fused by the radiation heat, and fixed onto the copying paper.
In the oven fixing system, a powder toner image, transferred onto copying paper from a photosensitive member or an intermediate transferring member, is irradiated with infrared rays under an oven atmosphere so that the toner image is fused by the radiation heat, and fixed onto the copying paper.
These non-contact heat fixing systems have the following superior features.                Since the powder toner image is fused and fixed without contacting any members, the toner image is free from damages caused by those members so that upon developing, there is no degradation in the resolution.        Since the fixing time is very short, a high-speed fixing process is available.        Since no waiting time is required for the fixing process, it is possible to start the process quickly.        These systems are readily applied to various kinds of copying paper having different thicknesses and qualities.        
However, since the non-contact heat fixing system carries out a heat fixing process in a non-contact state, its ambient energy dissipation is great. From the environmental viewpoint, there have been demands for a reduction in fixing energy. Even under such circumstances, when the total amount of light energy to reach the powder toner image is insufficient, the powder toner image is not sufficiently fused, resulting in the problem of insufficient fixing characteristics. In particular, in the case of a full-color image in which black images and color images are simultaneously printed, since the amount of energy to be absorbed is different depending on the respective colors, it is very difficult to control the amount of energy to be applied.
For this reason, in order to achieve a sufficient melt-fixing process, for example, the following toners have been proposed: a color toner containing at least a binder resin, an infrared absorbing agent, a colorant and a specific ester compound (for example, see Japanese Patent Application Laid-Open No. 2001-22,127 (page 2), Japanese Patent Application Laid-Open No. 2001-92,174 (page 2)), and a flash-fixing toner, which relates to a flash-fixing toner that is used for an image having toners of three colors or more laminated thereon, and contains an infrared absorbing pigment the absorbance in 650 nm of which is set to not more than 10% of the absorbance in the spectrum absorbing maximum wavelength in the infrared range, and which is characterized by containing at least two kinds of infrared absorbing pigments which have absorbing maximum wavelengths that deviate from each other by not less than 20 nm (see Japanese Patent Application Laid-Open No. 2002-99,111 (page 2)).
However, the above-mentioned toner has failed to provide sufficient color fixing properties depending on types and fixing conditions of images. For example, in the case when the fixing energy is comparatively small, even if desired color reproducibility is obtained in the case of a solid image having a great amount of toner adhesion, there is degradation in the color reproducibility when an image having a comparatively small amount of toner adhesion, such as a character image and a half-tone image, is fixed. This problem is more conspicuous in a half-tone image than in a character image, when the amount of toner adhesion is the same. In the case of a (full) color image including full-color portions and mono-color portions, the mono-color portions (except for black portions) that have a smaller amount of toner adhesion tend to have degradation in the fixing property. In order to improve the fixing property, when the amount of energy exerted on the fixing property is increased, the energy absorption becomes excessive in the black toner portions, causing a bumping phenomenon and the resulting noise. In the case when copy paper bearing an image formed on at least one surface thereof is fed, the fixed image tends to be rubbed against a roller or the like to cause degradation in the image quality such as blurring or stains (degradation in the smearing preventive property).